1. Field of the Invention
The invention relates to electrical switchboard apparatus and more particularly to switchboards having isolated bus systems.
2. Description of the Prior Art
An electrical distribution switchboard distributes the main incoming power among various site functions such as heating, lighting, and air conditioning. It typically consists of a number of vertical cabinet sections containing circuit breakers for switching and protecting the various load circuits. Three-phase electrical power enters the switchboard via cable or bus duct connected to a main bus which runs horizontally within the switchboard between the various sections. The main bus is connected to vertical, or riser, bus in each vertical cabinet section. Various circuit breakers stacked vertically within the cabinet have their inputs connected to the vertical riser bus and their outputs connected to horizontal load side extensions which run toward the rear of the cabinet perpendicular to the main horizontal bus. Bus duct or cable is then attached to the load side extensions to permit the power to exit the switchboard and flow to the load (air conditioning, lights, motors, etc.).
The primary requirements of a switchboard are that it be safe and dependable and that it exhibit low cost in construction, installation, and maintenance. Insulation in most switchboards is provided by physical separation of the various components at high potential, according to the requirements of Underwriters Laboratories. However, it is often desired to protect and isolate components at high potential from contact with tools, dust or other contaminating material, and pests such as insects or rodents.
Early investigations of switchgear bus systems have demonstrated that when bus insulation is in intimate physical contact with major surfaces of the conductors, the insulated bus will run at a cooler temperature than bare bus. Various methods of providing such contact are known in the prior art, such as coating the bus bar conductors using a fluidized bed process or an electrostatic spray painting process. Taping of the individual conductors will also provide very tight insulation. However, many disadvantages exist with these methods, such as relatively expensive capital equipment investment, difficulty of field installation or repair, and difficulty of applying insulation to a welded assembly. Glass polyester moldings have also been used to provide isolation for prior art switchboards. Although the actual material cost of glass polyester is moderate, the tooling and labor costs required for injection molding of this material are very high.
It is therefore desirable to provide an electrical switchboard having a protective insulation/isolation system which requires minimum tooling cost, a minimum number of parts, low assembly costs, favorable heat transfer characteristics, and minimum capital equipment and facilities investment.